This invention relates to a method for producing high purity methacrylic acid esters in high yield. A number of commercial processes are practiced for the production of such esters including sulfuric acid treatment of acetone cyanohydrin, two stage oxidation of isobutylene or t-butyl alcohol, and liquid phase catalytic condensation of propionaldehyde with formaldehyde.
A typical commercial cyanohydrin process is described in U.S. Pat. No. 4,529,816 ("816") for the production of methyl methacrylate ("MMA"). In this process, acetone cyanohydrin ("ACH") is (1) hydrolyzed by sulfuric acid to produce .alpha.-hydroxyisobutyramide ("HIBAM") and its sulfate ester, .alpha.-sulfatoisobutyramide ("SIBAM"); (2) the HIBAM and SIBAM are thermally converted to 2-methacrylamide ("MAM") and a small amount of methacrylic acid ("MAA"); which is then (3) esterified with methanol to produce MMA (residual HIBAM is esterified to methyl .alpha.-hydroxyisobutyrate ("MOB")). Also disclosed in '816 is an improvement to the general process for producing MMA wherein the MOB present is isolated and returned to the process between the thermal conversion and esterification steps.
The thermal conversion step in '816 is conducted at a sufficiently high temperature and for a sufficiently long time to maximize MAM formation from HIBAM and SIBAM prior to esterification. The thermal conversion of SIBAM is more facile than the thermal conversion of HIBAM. One objective of '816 is to obtain the maximum yield of MAM and MAA at this step. MAA is a co-product of the thermal conversion step and is also converted to MMA during esterification. In addition, '816 specifies that care must be taken when returning isolated MOB into the process between the thermal conversion and esterification steps such that the sulfuric acid strength is maintained at about 97-100% and the mole ratio of free sulfuric acid to MOB is maintained between 4 and 40.
European Patent Application No. 407,811 A2 discloses an acetone cyanohydrin process that does not require sulfuric acid for preparing methacrylic acid esters. A key step of this process is the vapor phase dehydration/dealcoholization of .alpha.-hydroxy-, .alpha.-alkoxy, or .beta.-alkoxy carboxylic acid esters in the presence of an alkali metal and platinum group element modified crystalline aluminosilicate disclosed in European Patent Application 429,800 A2. However, this process suffers from low yields based on ACH.
Even with the improved process described in '816, there is still a need for a high purity, high yield MMA process. Part of the driving force for additional improvements is the need to reduce waste products from currently practiced manufacturing processes.